DON'T do this with 4! Do with only one more person. You'll have Mantis instead of warthogs and it will be incredibly easy.

Yes, using a Mantis is really easy, we didn't even die on legendary and had a lot of fun (after we died like 50 teams each chapter before).

The spartan ops 5.1 Achievo was kinda glitchy for me. Played 4 player matchmaking, I didn't die (but my whole team did), but I didn't get it. One generator survived. We had a 3 sec black screen in the begining, maybe that ruined it. Or somebody pushed this button too early, I don't know. I'll have a try later, can't ne that difficult to achieve.